In producing liquids, including oil and water, from a geological formation, most wells initially have sufficient natural bottom hole pressure to efficiently lift the liquids up to the ground surface. However, over a period of time, this natural bottom hole pressure declines, thus requiring artificial steps to improve lift. One commonly known method of augmenting lift is to inject gas into the production tubing. This injection is usually done by forcing gas down the annulus between the production tubing, which conducts liquid to the surface, and the casing of the well. The gas is constrained to flow through a gas flow control device at the desired depth into the production tubing. The gas bubbles mix with the liquids and reduce the overall density of the mixture. With the liquid's density reduced, the diminished natural bottom hole pressure is then able to lift the liquid to the surface. This injection of gas into the well requires the operation of a gas lift control valve that regulates the injection of gas flow into the tubing.
In conventional applications, various types of lifting gas injection control valves can be utilized. Among the simplest of these is the orifice valve, which consists of a specifically-sized orifice insert mounted in the valve body and a back-flow check. The size of the orifice used is normally chosen based on calculated or estimated parameters, and therefore may or may not prove to be optimal in the actual application. Furthermore, in order to confirm whether or not the chosen orifice size is optimal, it may be necessary to remove and replace the valve one or more times using different orifice sizes to compare well performance data. Each act of removing and replacing the valve requires an interruption of well production as well as a period of time for the well to re-stabilize before useful comparative production data can be obtained. Additionally, an artificial-lift well whose reservoir characteristics are of a transient nature may require regular changing of the lifting valve orifice in order to maintain optimal conditions. A significant disadvantage with this system is that several trips into and out of the hole have to be conducted to achieve the proper setting. These multiple trips are, of course, time consuming and costly.
The operating valve of an artificial lift installation is normally intended to regulate or restrict the flow of injection gas from the casing into the production tubing and allow the flow of injected in response to either a preselected pressure condition or control from the surface. A difficulty inherent in the use of gas lift valves which are either fully open or closed is that gas lift production completions are closed fluid systems which are highly elastic in nature due to the compressibility of the fluids and the frequently large depth of the wells. For this reason, and especially in the case of dual completion wells, the flow of injected gas through a full open gas lift valve may produce vibrations at a harmonic frequency of the closed system and thereby create resonant oscillations in the system generating extremely large and destructive forces within the production equipment. Gas lift valves of a particular size aperture positioned at a point of resonance within the well completions(s) may have to be replaced in order for the system to be operable.
Another application of downhole fluid control valves within a production well is that of chemical injection. In some wells, it becomes necessary to inject a flow of chemicals into the borehole in order to treat either the well production equipment or the formation surrounding the borehole. The introduction of chemicals through a downhole valve capable of only fully open or fully closed positions does not allow precise control over the quantity of chemicals injected into the well.
Another application of downhole flow control valves is that of a dual completion gas lift operation in a well. By varying the orifice size of the gas injection valve the differential pressure drop across the gas lift valve can be controlled so that the pressure of the gas inside each string of tubing at the injection valve can be matched with the needs of that particular formation. However, flow control valves capable of only fully open or closed configurations contribute to imprecise control over the pressure drop. In addition, such systems also suffer from potential resonance due to oscillations generated by flow through the valve which may necessitate tuning the system in some fashion or replacement of the valve in order for the system to be operable.
Yet another application of downhole fluid control valves is in "auto lifting" applications. Auto lifting occurs where gas from one geological formation at a relatively higher pressure is used to supply the lifting energy to the liquids from a separate formation, all within the same wellbore.
As mentioned above, prior art flow control valves for downhole applications, such as gas lift valves, include a number of inherent disadvantages. A first disadvantage is having a single size flow orifice in the open condition which may produce resonant oscillations resulting in destructive effects within the well. A second disadvantage is that of being capable of assuming only a fully open or fully closed position which requires the shuttling of the valve between these two positions at high pressures and results in tremendous wear and tear on the valves. Such wear requires frequent maintenance or replacement of the valves which is extremely expensive.
Another type of valve utilized in gas lift applications is a hydraulic actuated valve that is generally controlled from the surface. By controlling the flow of a hydraulic fluid from the surface, a poppet valve is actuated to control the flow of fluid into the gas lift valve. The valve is moved from a closed position to an open position for as long as necessary to effect the flow of the lift gas. Such valves are also position instable, that is upon interruption of the hydraulic control pressure, the gas lift valve returns to its normally closed configuration. Other hydraulically actuated downhole flow control valves also include certain inherent disadvantages as a result of their long hydraulic control lines which result in a delay in the application of control signals to a downhole device. For example, in applications involving hydraulically driven motors or pistons, the precise flow of hydraulic fluid necessary to adjust the valve to essential critical tolerances is often difficult to achieve due to the hysteresis that develops in a hydraulic system that spans substantial well depths. Another complicating factor is the hydraulic head that is present at those same well depths. At such depths, the pressure attributable to the head of hydraulic fluid can become quite significant, which makes the setting of the valve more difficult because of the added hydraulic pressure that must be compensated for when adjusting the valve. These problems exist primarily because the point of fine tuning for the valve depends on the flow of hydraulic fluid that is controlled from the distant surface, and for the reasons stated above, fine tuning adjustment to the valve is difficult to achieve.
To overcome some of the above-described disadvantages, electrically controlled gas lift valves have been developed. However, some of these valves, such as the one disclosed in U.S. Pat. No. 3,427,989, also suffer from disadvantages of position instability and operation based upon either fully open or fully closed conditions. Another electrical valve, which is disclosed in pending U.S. application, Ser. No. 08/218,375 and is incorporated herein by reference, addresses many of the problems suffered by prior electrical control valves by providing an electrical valve that allows the adjustment of a variable orifice size valve by means of signals from the surface. While these valves are well suited for their intended use, they are more expensive and complex in their design than the conventional hydraulic valves discussed above.
Thus, it is apparent that there is a need in the art for an inexpensive, simply designed, fluid actuated control valve in which the orifice size of the valve is adjustable through a range of values that would enable gas lift systems, which are susceptible to resonant oscillation, to be detuned by adjusting the size of the orifice to dissipate the resonant oscillations. Such a variable orifice valve would allow much greater control over the quantity and rate of injection of fluids into the well. In particular, more precise control over the flow of injection gas into a dual lift gas lift well completion would allow continuous control of the injection pressure in both strings of tubing from a common annulus, which would result in more efficient production from the well.
There is also a need in the art for a fluid actuated control valve that would be position stable; that is, it would be able to set a flow control valve at a particular orifice size and to have it remain at that same orifice size until selectively changed to a different size without the need for well intervention to change the orifice size, i.e., pulling the valve. There is also a need in the art for a fluid actuated control valve that is able to monitor not only the orifice size of the valve but also the pressures and flow rates within the production system in order to obtain desired production parameters within the well.
The fluid actuated flow control valve system of the present invention provides a valve system that addresses the deficiencies of the prior art valves.